Hydrophobized gas diffusion layers and method of making the same

ABSTRACT

A gas diffusion layer having a first major surface and a second major surface which is positioned opposite to said first major surface and an interior between said first and second major surfaces is formed. The gas diffusion layer comprises a porous carbon substrate which is directly fluorinated in the interior and is substantially free of fluorination on at least one of the first major surfaces or the second major surfaces, and preferably both surfaces. The gas diffusion layer may be formed using protective sandwich process during direct fluorination or by physically or chemically removing the C—F atomic layer at the major surfaces, for example by physical plasma etching or chemical reactive ion etching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a Continuation of U.S. patentapplication Ser. No. 14/372,824 filed on Jul. 17, 2014, which is a 371Application of PCT/US2013/022036 filed on Jan. 18, 2013, which claimspriority to U.S. Provisional Application No. 61/591,357 filed on Jan.27, 2012, each of which is incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.EFRI-1038234 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

STATEMENT REGARDING JOINT RESEARCH AGREEMENT

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to gas diffusion layers (“GLDs”), such asthose which may be useful in the manufacture of fuel cells.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a gas diffusion layer for use in agas diffusion device, such as a proton exchange membrane fuel cell. Thegas diffusion layers are designed to be positioned adjacent to acatalyst layer in the fuel cell. The gas diffusion layer exhibitselectron conductivity and gas diffusibility. The gas diffusion layer hasa first major surface and a second major surface which is positionedopposite to the first major surface and having an interior between thefirst and second major surfaces. The gas diffusion layer comprises aporous carbon substrate which is directly fluorinated in the interiorand is substantially free of fluorination on at least one of the firstmajor surfaces or said second major surfaces.

In one aspect, at least 50 percent of the surface area of the majorsurface that is designed to be in contact with the catalyst layer is notfluorinated. For example, at least about 50, 55, 60, 65, 70, 75, 80, 85,90, 92, 94, 96, 98, or 100% (or some range therebetween) of the surfacearea of the major surface that is designed to be in contact with thecatalyst layer is not fluorinated. As another example, at least about50, 55, 60, 65, 70, 75, 80, 85, 90, 92, 94, 96, 98, or 100% (or somerange therebetween) of the surface area of both major surfaces is notfluorinated.

In another aspect, the gas diffusion layer contains less than about 2 at% fluorine. For example, the amount of fluorine in the gas diffusionlayer is less than about 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7,0.6, 0.5, 0.4, or 0.3 at % (or some range therebetween).

In another aspect, the gas diffusion layer contains less than about 2 wt% fluorine. For example, the amount of fluorine in the gas diffusionlayer is less than about 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7,0.6, 0.5, 0.4, or 0.3 wt % (or some range therebetween).

In another aspect, the gas diffusion layer has a mean pore size afterdirect fluorination of about 100 microns or less (e.g., about 100, 90,80, 70, 60, 50, 40, 30, 20, 10 microns or less).

In another aspect, the present invention is directed to a fuel cell ordevice containing the gas diffusion layers of the present invention. Thefuel cell comprises a membrane-electrode assembly including a hydrogenion conductive polymer electrolyte membrane; a pair of catalyst layerssandwiching the polymer electrolyte membrane; and a pair of gasdiffusion layers of the present invention disposed on the outer surfacesof the catalyst layers.

In another aspect, the present invention is directed to a method formaking the gas diffusion layer. The method comprises the steps ofproviding a porous carbon substrate having a first major surface and asecond major surface which is positioned opposite to the first majorsurface and having an interior between the first and second majorsurfaces; protecting at least one of the first major surface and thesecond major surface by contacting at least one surface with aprotective layer; directly fluorinating the porous carbon substrate withfluorine gas; and removing the protective layer from at least onesurface. The interior of the gas diffusion device is fluorinated and atleast one major surface (preferably both major surfaces) is notsubstantially fluorinated.

In still another aspect, the direct fluorinating step occurs at atemperature of about 300 to 500° C. (e.g., about 300, 310, 320, 330,340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470,480, 490, or 500° C. or some range therebetween).

In yet another aspect, the direct fluorinating step occurs using amixture of fluorine gas and an inert gas. The inert gas is preferablyselected from the group consisting of argon and nitrogen. The amount offluorine in the gaseous mixture is preferably between about 1 and 50%(e.g., about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50% or some rangetherebetween). In one aspect, the direction fluorinating step includesusing a mixture of about 10 to 20% fluorine and 80 to 90% argon.

In a further aspect, the direct fluorinating step occurs at atmosphericpressure or an elevated pressure. Exemplary pressures include 1, 2, 3,4, or 5 atm.

In another aspect, the direct fluorinating step occurs for about 10minutes to 5 hours (e.g., about 10, 20, 30, 40, 50, 60, 90, 120, 150,180, 210, 240, 270, or 300 minutes or some range therebetween).

In yet another aspect, during the direct fluorinating step, the firstmajor surface is protected by contacting the first major surface with afirst protective layer and the second major surface is protected bycontacting the second major surface with a second protective layer suchthat the porous carbon substrate is sandwiched between the firstprotective layer and the second protective layer.

In another aspect, during the direct fluorinating step, the protectivelayer(s) are comprised of the same material comprising the porous carbonsubstrate. The protective layers may also be comprised of a differentmaterial, provided that protective layers are capable of contactingsubstantially all of the first or second major surfaces and are gaspermeable. Theoretically, any porous materials that can withstand thetemperature and fluorination environment, like porous glass, quartz, orTeflon, may be used.

In an alternative embodiment, the major surface(s) of the porous carbonsubstrate are not protected during the direct fluorination step. Thus,during the direct fluorination process, the outer carbon atomic layer ofthe carbon fibers are fluorinated. In this embodiment, the outer carbonatomic layers of the carbon fibers forming at least one of first andsecond major surfaces (preferably both surfaces) are removed such thatthe interior of the gas diffusion device is fluorinated and at least onemajor surface (preferably both major surfaces) is not substantiallyfluorinated. The removing step may be performed by physical or chemicalmethods, for example sanding, physical plasma etching with an argonplasma, or reactive ion etching.

In another aspect, the gas diffusion layer may be comprised of a singlelayer of material or a plurality of layers. In one aspect, the gasdiffusion layer is a bilayer comprising a macroporous substrate and amicroporous substrate. For example, the gas diffusion layer may comprisea macroporous carbon substrate adjacent to a microporous carbonsubstrate (which is typically comprised of carbon powder mixed withpolytetrafluoroethylene (“PTFE”)). The gas diffusion bilayer may bedirectly fluorinated using the methods discussed herein (e.g., asandwich method or physical/chemical removal of the outer carbon atomiclayers, such as by sanding). In such an embodiment, the gas diffusionlayer is a bilayer having a first major surface comprised of themicroporous carbon substrate and a second major surface comprised of themacroporous carbon substrate which is positioned opposite to the firstmajor surface and having an interior between the first and second majorsurfaces. The gas diffusion layer comprises a porous carbon substratewhich is directly fluorinated in the interior and is substantially freeof fluorination on at least one of the first major surfaces or secondmajor surfaces. In another aspect, at least 50 percent of the surfacearea of the major surface of the microporous carbon substrate that isdesigned to be in contact with the catalyst layer is not fluorinated.For example, at least about 50, 55, 60, 65, 70, 75, 80, 85, 90, 92, 94,96, 98, or 100% (or some range therebetween) of the surface area of themajor surface of the microporous layer is not fluorinated. As anotherexample, at least about 50, 55, 60, 65, 70, 75, 80, 85, 90, 92, 94, 96,98, or 100% (or some range therebetween) of the surface area of bothmajor surfaces (macroporous and microporous substrates) is notfluorinated.

Additional aspects of the invention, together with the advantages andnovel features appurtenant thereto, will be set forth in part in thedescription which follows, and in part will become apparent to thoseskilled in the art upon examination of the following, or may be learnedfrom the practice of the invention. The objects and advantages of theinvention may be realized and attained by means of the instrumentalitiesand combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an internal structure of a typicalproton exchange membrane fuel cell (“PEMFC”).

FIG. 2 illustrates the three-layer sandwich and one-layer setup formaking the gas diffusion layer of the present invention.

FIG. 3 is a schematic view, illustrating the experimental apparatus usedto make the gas diffusion layer of the present invention using directfluorination.

FIG. 4 is a photograph of the sandwich apparatus used to make the gasdiffusion layer of the present invention using direct fluorination.

FIG. 5 are photographs illustrating the contact angle of the gasdiffusion layer of the present invention. A higher contact anglegenerally means higher hydrophobicity.

FIG. 6 shows the contact angle as a function of treatment temperature.The first number shown is the treatment temperature and the secondnumber is the water droplet contact angle.

FIG. 7 shows the EDX results of a gas diffusion layer of the presentinvention using material which has been crushed for analysis.

FIG. 8 illustrates an experimental set up for measuring the capillarypressure of the gas diffusion layer of the present invention.

FIG. 9 is an SEM of a gas diffusion layer comprising a bilayer of amacroporous carbon substrate and a microporous carbon substrate that hasbeen fluorinated in accordance with the present invention.

FIG. 10 illustrates the test results of two fuel cells in which the oneis made of a commercial bilayer gas diffusion material and that theother is made of a directly fluorinated bilayer gas diffusion materialin accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A fuel cell is basically a generator apparatus that uses the reversereaction of the water electrolysis to convert chemical energy intoelectric energy. Since the fuel cell has the advantages of low operationtemperature, quick start, high energy density, low pollution, and a widerange of applications, the fuel cell has a high commercial value. It hasbecome a successively developed and promoted technology all over theworld. The commonly seen fuel cell includes the phosphoric acid fuelcell, direct methanol fuel cell, alkaline fuel cell, molten carbonatefuel cell, solid oxide fuel cell, and proton exchange membrane fuelcell.

FIG. 1 is a cross-sectional view of an internal structure of a typicalproton exchange membrane fuel cell (“PEMFC”). As shown in FIG. 1, thePEMFC mainly includes a proton exchange membrane 100, catalyst layers102, gas diffusion layers 104, and bipolar plates 106. During the PEMFCoperation, the oxidation reaction of H2 is taking place at the anode110, and the reduction reaction of O2 is taking place at the cathode112. The reactant gas H2 at the anode 110 is decomposed into hydrogenions (H+) and electrons (e−) in the presence of a catalyst. Theelectrons (e−) escape from the anode 110, flow through the cell externalcircuit 114 and load 115, then reach the cathode 112. Meanwhile, thehydrogen ions (H+) are transferred from the anode 110 to the cathode 112through the proton exchange membrane 100. The hydrogen ions (H+) and theelectrons (e−) combine with the oxygen molecules (O2) at the cathode 112to produce water (H2O).

During the fuel cell reaction, the H2O molecules will be continuouslytransferred from the anode 110 to the cathode 112 by electroosmoticdrag. If the water cannot be supplied at adequate amount to the anode,the proton exchange membrane 100 will become excessively dry, the H+conducting capability of the membrane will be reduced, and the poweroutput of the fuel cell will be significantly reduced. Meanwhile, agreat amount of water (H2O) will be produced by the oxygen reductionreaction (“ORR”) at the cathode 112. If the water transferred from theanode to the cathode by electroosmotic drag and produced by ORR cannotbe adequately discharged from the cell, the catalyst layer 102 and thegas diffusion layer 104 at the cathode 112 will be flooded with water.The gas diffusion layer filled with water becomes a diffusion barrier ofoxygen. It retards the oxygen getting into the catalyst layer and thecell output power is significantly reduced. Therefore, controlling andmaintaining the water balance in the cathode 112 and the anode 110 andkeeping the gas transferring freely inside the electrodes, are criticalfor maintaining the performance of the PEMFC at its optimal condition.

The gas diffusion layer 104 is located between the catalyst layer 102and the gas flow path. At the catalytic sites on each electrode, it isthe gas diffusion layer 104 that provides both a path of electricalconduction and passage for reactants and products, such as hydrogen,oxygen, and water. That is, the gas diffusion layer is required to havehigh reaction gas permeability, high water permeability, and highelectron conductivity.

The gas diffusion layer 104 must be porous in nature. The word porousgenerally refers to the volume of interstices of a material relative tothe volume of the mass of the material. Porosity effects the state ofpermeability of a material, that is the property of a porous materialthat is the measure of the amount (rate or volume) at which a fluid(liquid or gas) passes through a unit of cross-section of material at agiven viscosity, under a unit of gradient pressure. Therefore, at fixedgradient pressure, and viscosity, the permeability of a given materialis directly related to its porosity. For purposes of this application,therefore, the terms porosity and permeability may be usedinterchangeably with the understanding that an increase in porosity(interstitial volume) will normally result in an increase inpermeability, and vice versa.

Any suitable gas diffusion layer material may be used in the practice ofthe present invention. Typically the gas diffusion layer is comprised ofsheet or roll good material comprising carbon fibers. Typically, the gasdiffusion layer is a carbon fiber construction selected from woven andnon-woven carbon fiber constructions. Carbon fiber constructions whichmay be useful in the practice of the present invention may include:Toray™ Carbon Paper, SpectraCarb™ Carbon Paper, Zoltek™ Carbon Cloth,AvCarb™ P50 carbon fiber paper, and the like. The gas diffusion layermay be provided in sheets, as a roll good, or in any suitable form.

As shown in FIGS. 1 and 2, the gas diffusion layer 104 has a first majorsurface 140 and an opposite second major surface 150. The thickness ofthe gas diffusion layer is typically between about 10 and 500 microns,more preferably between about 30 and 300 microns (e.g., about 30, 50,70, 100, 130, 150, 200, 250, or 300 microns).

In order to prevent the gas diffusion layer from becoming flooded withliquid water in the present invention, the gas diffusion layer materialis made more hydrophobic by direct fluorination. However, it has beenshown that fluorination increases the ohmic resistance of carbonmaterials, mainly due to the C—F product on the surface (see Table 1).Thus, if the surface of the gas diffusion layer is in contact with othercomponents like the catalyst layers and flow distributor plates, theresistive C—F surface may lead to higher ohmic resistance in the celland poorer performance.

TABLE 1 Electronic conductivity of different graphite fluorides StartingComposition graphite CF (HT) CF_(0.8)I_(0.02) CF_(0.65)I_(0.05)CF_(0.52)I_(0.06) Electrical 360 <10⁻¹² ~10⁻⁷ ~10⁻⁶ ~10⁻¹ conductivityσ: Ω⁻¹cm⁻¹ Note: HT denotes high temperature.

In the present invention, the conductivity of the gas diffusion layer104 is maintained because at least one of the first major surface 140and the second major surface 150 is substantially free of fluorination.As used therein, the term “substantially free” means that at least 50percent of the surface area of the major surfaces (e.g., the surface 140designed to be in contact with the catalyst layer) is not fluorinated.For example, at least about 50, 55, 60, 65, 70, 75, 80, 85, 90, 92, 94,96, 98, or 100% of the surface area of the major surface (e.g., thesurface 140 that is designed to be in contact with the catalyst layer)is not fluorinated. Most preferably, both major surfaces 140, 150 of thegas diffusion layer 104 are substantially free of fluorination. Thedegree to which the major surfaces 140, 150 of the gas diffusion layer104 are fluorinated may be the same or different.

In one aspect, the first and/or second major surfaces 140, 150 of thegas diffusion layer 104 are protected from direct fluorination duringthe fluorination process. The surfaces may be protected by contactingthe surface with a protective layer during the fluorination process. Inan exemplary aspect, the first and second major surfaces 140, 150 areprotected by sandwiching the gas diffusion layer 104 between twoprotective layers 160, 170 as generally shown in FIG. 2 (left panel). Ina preferred aspect, the protective layers are comprised of the samematerial used to form the gas diffusion layer 104. That is, the gasdiffusion layer 104 is sandwiched between two protective porous carbonsubstrates 160, 170. In one aspect, the contact involves a compressiveforce, although that is not required. The porous protective gasdiffusion layers 160, 170 on the top and bottom of the gas diffusionlayer 104 permit the fluorine gas to penetrate and react with the gasdiffusion layer 104 in the middle while protecting its outside majorsurfaces 140, 150. The fluorination may be performed in either a batchor continuous process. In a continuous process, the protective layers160, 170 continuously contact the gas diffusion layer 104, and may beunder compression.

In another aspect, the present invention is directed to a one-layermethod for forming the gas diffusion layer. In this method, the gasdiffusion layer 104 is directly fluorinated by exposure to fluorine gas,as generally shown in FIG. 2 (right panel). In general, the gasdiffusion layer 104 is typically comprised of carbon fibers having tensof thousands of carbon atomic layers (i.e., typically micron-sizedcarbon fibers). Direct fluorination results in a C—F covalent bond atthe top atomic layer of the carbon atoms (i.e., the internal carbons ofthe carbon fiber are not fluorinated). If this upper/top atomic C—Flayer is removed, the underlying carbon substrate of the carbon fibersmay be exposed. Thus, in this embodiment, the fluorinated outside majorsurfaces 140, 150 of the gas diffusion layer 104 are removed by aphysical or chemical method, for example by sanding, physical plasmaetching, or chemical reactive ion etching. Physical etching is typicallydone using an argon plasma. The energetic argon atoms in the plasmaremove both the fluorine and carbon atoms on the upper/top atomic C—Flayer. The extent of the removal will depend upon the power level andthe duration of the etching, and can be readily determined by oneskilled in the art. Likewise, chemical reaction ion etching uses plasmaand reactive gases like SF6 which can etch the upper/top atomic C—Flayer by both physical etching and by reacting with the surface to froma gaseous product. Thus, in one aspect, the upper/top C—F layer isremoved by an etching method selected from the group consisting of ionetching, ion beam etching, reactive ion etching, reactive ion beametching, and combinations thereof. The plasma excitation can for examplebe performed by supplying microwave electric power, radio frequencyelectric power, or DC electric power.

The one-layer approach may be used as a batch or continuous process inwhich there can be two stages. In the first stage, the porous gasdiffusion materials 104 can be fed continuously from a roll into adirect fluorinating reactor chamber. After the material comes out of thefirst stage, it can be fed continuously into the second stage in whichthe C—F layers of the outer major surfaces 140, 150 can be removed. Thisalternative method may be more economical than the sandwich approach,which is generally more suitable for batch processes.

EXAMPLE 1

Experimental Setup:

Pure fluorine gas (F2) was used to react with the gas diffusion layersat temperatures ranging from 400° C. to 550° C. A schematic view of theexperimental setup is shown in FIG. 3. The fluorine gas 10 (preferablyabout 85% argon and 15% fluorine) is introduced into atemperature-controlled nickel reactor 20 at about 1 atm. Prior to andfollowing the fluorination, sufficient nitrogen is supplied as purge gasto flush the air and fluorine out of the nickel reactor. To preventcorrosion of the thermocouple, nickel coated thermocouples are used. Twothree-way valves 30 are used to select the flowing gas at differentstages of the experiment. A mass flow controller controls the fluorineflow rate. Nickel is preferably used for all parts in contact withfluorine at high temperatures. Other parts of the connections underambient temperature can be stainless steel. Monel is inert in fluorinebelow 550° C.

Fluorination is preferably carried out at temperatures above about 350°C., for example about 400° C., 450° C., 500° C., and 550° C. andpressures of about 1 to 5 atm.

Bubbling the exhaust gas through sodium hydroxide solution 40 before itis released into the air captures the un-reacted fluorine gas, by whichthe highly toxic gas is fully absorbed. The sodium hydroxide solutionreacts with fluorine according to the following reactions:2F₂+2NaOH→OF₂+2NaF+H₂OOxygen difluoride slowly reacts with water to form fluoric acid:OF₂(aq)+H₂O(aq)→2HF(aq)+O₂(g)In a basic solution, the fluoric acid is immediately neutralized by thesodium hydroxide.HF+NaOH→NaF+H₂OThus, the overall reaction may be rewritten as:2F₂+4NaOH→4NaF+2H₂O+O₂In this way, the sodium hydroxide prevents the corrosive fluorine gasfrom being emitted directly into the air. Alternatively, a closedfluorine recirculation loop (i.e., essentially a semi-batch process)could be used to eliminate fluorine emission during operation, andeffluent treatment is only needed at the beginning and at the end of theexperiment when the gas in the reactor is purged.

Experimental Procedure:

Direct fluorination is carried out according to the following steps: (1)Flush the nickel reactor with an inert gas (e.g., nitrogen or argon) forabout one hour to replace the air in the reacting chamber; (2) Raise theoven temperature to desired value (typically about 300 to 450° C.) untilstabilized while continuously flowing the nitrogen to the nickelreactor; (3) Flow fluorine gas into the nickel reactor for about one tothree hours to fluorinate the gas diffusion layer in the reactor, andstop heating the nickel reactor when the fluorination reaction iscompleted; (4) Turn off heater and allow the reactor to cool to roomtemperature; (5) Switch to the inert gas (e.g., nitrogen or argon) toflush the remaining fluorine from the reactor; and (6) Allow to cool toroom temperature before removing the gas diffusion layer from the ovenand disassembled to remove the samples.

An exemplary three-layer gas diffusion sandwich apparatus 50 isillustrated in FIG. 4. The gas diffusion layer is placed between twostainless steel plates 60, 62 with slots or openings 66 machined intothe plates to distribute the fluorine gas. The edges are sealed with acopper gasket 68.

Gas Diffusion Layers:

Commercially available gas diffusion layers with different propertiesfrom two different companies (SGL and Toray) will be used in theexperiment. See Table 2. These gas diffusion layers are often used inPEM fuel cells. The gas diffusion layer with 0% PTFE will be treatedwith direct fluorination to validate the hypothesis that making a porousmedium hydrophobic by modifying its surface property will cause thecapillary pressure (defined here to be the gas pressure, P_(gas), minusthe liquid pressure, P_(liquid)) to move downward, as compared to movingleftward when it is treated by the pore-filling physical coating method.It will also allow the quantifying separately the effect of the surfacetreatment on the capillary pressure curve.

TABLE 2 Gas diffusion layers to be evaluated in the experiment ThicknessPTFE Company GDLs (μm) Porosity (wt. %) SGL Group^(a) 24 AA 200 0.84 024 BA 200 0.84 5 24 CA 200 0.84 10 24 DA 200 0.84 20 24 DC* 200 0.84 2024 EA 200 0.84 30 24 EC* 200 0.84 30 Toray^(b) TGP-H-060 190 0.8 0TGP-H-060 190 0.8 10 TGP-H-060 190 0.8 20 TGP-H-060 190 0.8 30 TGP-H-060190 0.8 40 ^(a)From SGL Group product data sheets. ^(b)From TorayIndustries, Inc. product data sheets. *With microporous layer (MPL) onone side

Characterization Methods:

Contact Angles

The gas diffusion layers were tested by measuring its outerhydrophobicity using the water contact angle formed between a liquidwater droplet and the treated sample outer surface. High hydrophobicityis represented by high contact angle as shown in FIG. 5. The temperatureand time that result in the highest contact angle (highesthydrophobicity) are selected. While this test cannot measure thehydrophobicity or contact angle inside the sample, the method issuitable because since all exposed carbon surface reacts with fluorinethe inner exposed carbon surface should have the same property as theexposed outer surface.

As shown in FIG. 6, from the contact angle results obtained at differenttemperatures and reaction times, the optimal temperature range and timefor the fluorine gas composition and pressure used was determined to beabout 350 to 400° C. and about one hour. While shorter or longer timesare possible, a one hour reaction time is currently selected to ensureall exposed carbon surfaces are fluorinated.

SEM

Gas diffusion layers with physical coating will be cross-sectioned andexamined by SEM to determine the distribution characteristics of thePTFE phase in the porous structures. Of special interest is theinterconnectivity and continuity of the PTFE phase needed for optimalgas transport. After fluorination, the surface structure of carbonsubstrate will be examined by SEM to determine the coatingcharacteristics. Note that the carbon fibers in the gas diffusion media(“GDM”), are in the order of tens of micrometers making their surfaceeasily observable by high resolution SEM.

As shown in FIG. 7, SEM and EDX (Energy Dispersion X-Ray Spectroscopy)were used to analyze the samples to see whether there were any physicalchanges in the surface and fluorine on the carbon surface that could bedetected. The results show that direct fluorination does not change thephysical dimensions or shape of the materials. Since the amount offluorine is quite small, one fluorine atom per carbon on the surface ofthe carbon fibers which is a tiny fraction of the total carbon in thematerial, it is not detectable by EDX. However, as shown in the Table 3below, when the material is crushed to expose the surface inside thesubstrate the amount of fluorine in the sample is detectable. However,the amount is very small, about 0.2 atomic percent.

TABLE 3 Elemental Analogs of GDL (inventive) Elements Wt % At % C 98.0598.56 O 01.63 01.23 F 00.33 00.21For comparison, the results of a commercially PTFE treated carbon gasdiffusion material are shown in Table 4 below. Note that amount offluorine is as much as 25 atomic-percent or 35 weight-percent in thismaterial.

TABLE 4 Elemental Analogs of GDL (prior art) Elements Wt % At % C 64.9574.56 F 35.05 25.44

Porosity and Pore Size Distribution

The porosity and pore size distribution of the physically treated,chemically treated, and physically and chemically treated GDM will bemeasured by mercury intrusion porosimetry. Although mercury isnon-wetting to carbon fibers, it penetrates into the porous GDM whenpressure is applied. Pore size information may be extracted from thecapillary data using the Young-Laplace equation:

$r = \frac{2\sigma_{{Hg}\text{-}{Air}}\cos\;\theta_{{Hg}\text{-}{Air}}}{p_{c,{{Hg}\text{-}{Air}}}}$where r is the pore radius, σ_(Hg-Air) the surface tension of themercury-air interface, θ_(Hg-Air) the contact angle of mercury on thecarbon graphite, and p_(c,Hg-Air) the capillary pressure, respectively.This technique will give the total porosity of the sample. A schematicfor performing this test is shown in FIG. 8.

To get the hydrophilic porosity, water porosimetry, which is verysimilar to the method used for capillary pressure curve measurement, canbe used. In this technique, water instead of mercury is used as theintrusion fluid. Once the hydrophilic porosity is known, the hydrophobicporosity may be calculated by ε_(phobic)=ε_(total)−ε_(philic).

Neutron Imaging

Neutron imaging will also be used during hydrophilic porosity andcapillary pressure measurements to elucidate how water imbibes anddrains from these samples and whether continuous gas pathways werecreated in the samples by the hydrophobic phase. Note that since neutronpenetrates carbon and PTFE but not water one would be able to seecontinuous gas channels from the region unoccupied by liquid water. Theneutron imaging facility at NIST is capable of carrying out the taskdescribed here.

Spectroscopy Analysis

Various spectroscopy techniques can be used to analyze the fluorinatedcarbon materials. These include infrared, Fourier-transform infrared,electron spin resonance, X-ray photoelectron, and Raman spectroscopy.Due to the nature of the sample, we will attempt to apply two majortechniques: X-ray photoelectron spectroscopy (“XPS”) and laser Ramanspectroscopy (“RS”).

X-ray photoelectron spectroscopy, also known as electron spectroscopyfor chemical analysis (“ESCA”), is an ultra-high-vacuum (“UHV”)technique for surface chemical compositions analysis. With XPS, it ispossible to identify the nature and the amount of fluorine containinggroups attached to the surface of the carbon substrates (carbon fibersin GDL) because of the fluorination processes. The fluorine content inthe GDL can be calculated based on the difference in weight before andafter fluorination. However, this fluorine content is actually the ratioof fluorine to total carbon, which may depart significantly from thesurface composite. The XPS result will directly reveal the surfaceelemental and chemical information of the coating. The fluorine tocarbon ratios could be obtained by integrating the intensities of XPS C1s and F 1s features. XPS spectra for a carbon sample before and aftertreatment by direct fluorination shown in FIG. 8 clearly illustrate theformation of the CF species on the carbon surface. The F/C ratio couldbe determined by the areas covered by F 1s and C 1s peaks. Assuming thefluorine species are chemically bonded to carbon, this will change theenergy of the core electrons of the bonded carbon, resulting in achemical shift. For example, chemical shifts of 4.7, 6.7, and 9.0 eV ofC 1s peak (about 285 eV) can be attributed to the surface group of CF,CF₂, and CF₃, respectively. With the current state of the art XPSsystem, an energy resolution of 0.5 eV can be achieved. High resolutionXPS spectra will be collected to investigate the chemical nature ofcarbon fluorine bond.

Raman Spectroscopy (“RS”) is an increasingly popular analyticaltechnique because extensive sample preparation and high vacuumenvironment are not required. A Raman spectrum can also be used toidentify the surface functional groups of the carbon materials.Typically, two classical bands, D mode (disorder) and G mode (graphite),result from defects on the carbon and the carbon-carbon stretching mode.The ratio (“R”) of the intensities of the D band to that of the G bandrepresents the structure and the degree of disorder of the carbonsurface. For fluorinated carbon materials, R increases with thefluorination temperature, which directly correlates with the extent offluorine compound formation on the carbon surface. For the un-treatedcarbon nano fibers (“CNFs”), the defects come mainly from the disorderededges of the nanotubes, which have a very small R ratio (smallI_(D)/I_(G)).

Raman analysis for most material-of-interest is relatively easy andstraightforward, requiring almost zero sample preparation. However, inthe practical Raman analysis of fine carbon powders and carbon fibers,sample burning is a major concern. This is because the fine carbonmaterials exposed under a focused laser spot for extended period of timewill tend to heat up and burn. Laser attenuation and forced samplecooling will be applied if necessary to circumvent this issue.

Fuel Cell Performance & Water Transport Across the Membrane

Electrodes with a catalyst loading of about 0.3-0.5 mg/cm² will be madeusing GDM that were physically treated, chemically treated, and bothphysically and chemically treated. To prepare a membrane electrolyteassembly (“MEA”) the electrodes will be hot-pressed onto a Nafion® 112membrane (1100 equivalent weight & 2 mil thickness) at 135° C. for 5minutes with a pressure of 33 lb/cm². The MEA will be tested in a fuelcell with serpentine flow fields 60° C. with hydrogen and air fullyhumidified at high stoichiometric flow rates to 1) magnify the liquidwater flooding effect and 2) minimize the effect of variation in the gasrelative humidity along the flow channels.

To determine the effectiveness of the liquid water pressure generationcapability of the modified gas diffusion materials at the cathode on theback transport rate of water from the cathode to the anode, the watercontent in the cathode and anode feed streams and effluents will becarefully controlled and accurately measured. Based on the water-balancemeasurements we can determine the net water transport rate across themembrane. An experimental setup similar to the one used by Karan et al.will be used.

GDM (GDL & MPL) Durability

Running the fuel cell performance testing continuously for a week orlonger could test the durability of the GDMs in fuel cells. However, thefuel cell itself may complicate the problem because the other parts ofthe fuel cell, particularly the catalyst layer (“CL”), may age overtime. Since the capillary pressure curve is probably the most importantproperty of the GDM, this property will be selected as the indicator ofthe GDM durability. In a PEM fuel cell, the three parameters that havethe greatest impacts on the durability of a GDM are temperature, water,and acidity (due to the leaching of the ionic groups from the membrane).To simulate the conditions in a PEM fuel cell and to shorten the testingtime, the GDM will be soaked in a 1 M sulfuric acid solution at 80° C.Samples will be removed at some fixed time intervals, washed in DIwater, dried, and measured for changes in the capillary pressureproperty.

Safety Issues

The fluorine content in the air is controlled because of its toxicityeven in trace amounts. In this experiment, the un-reacted fluorine willbe consumed by the sodium hydroxide solution. The operation will becarried out in a hood with sufficient venting. Since fluorine is inertto nickel up to 650° C. a nickel reactor will be safe for operationaltemperatures up to 550° C. Fluorine recirculation will be used duringtreatment to minimize the use of fluorine and need for fluorinetreatment in the effluent. During nitrogen purge, sufficient time willbe used to ensure that fluorine is completely removed from the reactorbefore the reactor is open and samples removed. An experiment will alsobe conducted to determine the amount and concentration of sodiumhydroxide and the residence time of the effluent gas in the scrubberneeded to completely treat all the fluorine gas.

EXAMPLE 2

Those skilled in the art will appreciate that a gas diffusion layer maybe comprised of a dense hydrophobic microporous layer added to thesurface of the macroporous layer. The microporous layer is in contactwith the catalyst layer in order to keep the CL/GDL interface free ofliquid water and/or create a high capillary pressure boundary to driveliquid water back to the anode. As with the macroporous layer, themicroporous layer is conventionally made wet-proof by adding PTFE.However, the microporous is conventionally made differently from themacroporous layer. While the macroporous layer is made of carbon fibers,which is later made wet-proof by coating the carbon fibers with PTFE,the microporous layer is made of carbon powder mixed with PTFE. The PTFEphase in the micro-porous layer is used both as a binder and as awet-proofing material. In this approach, complete carbon surfacecoverage with PTFE is not possible because it will render this layernon-electrically conductive.

In this example, the gas diffusion layer was a bilayer comprising amacroporous substrate 104 a and a microporous substrate 104 b as shownin FIG. 9. More specifically, the gas diffusion layer comprised anuntreated macroporous carbon substrate adjacent a microporous carbonsubstrate comprised of carbon powder mixed with PTFE commerciallyavailable from SGL Carbon (Product No. SGL 35AC). The mean pore size forthe macroporous layer was about 15 to 30 microns, and the mean pore sizefor the microporous layer was about 1-3 microns. The gas diffusionbilayer was directly fluorinated at about 350 to 450° C. using themethods discussed herein. The F2-treated bilayer was then placed on ahigh grade sandpaper (grade 1500) and lightly rotated a few times overthe sandpaper. This sanding step was repeated for the other side of thegas diffusion bilayer. The major surfaces were then wiped withisopropanol to clean the surfaces. The resulting gas diffusion layer wasthus a bilayer having a first major surface comprised of the microporouscarbon substrate and a second major surface comprised of the macroporouscarbon substrate. The gas diffusion layer was directly fluorinated inthe interior and was substantially free of fluorination on the firstmajor surfaces and second major surfaces.

Test results for two fuel cells in which the cathode in one is made of acommercial bilayer gas diffusion material and that in the other is madeof directly fluorinated bilayer gas diffusion material are shown in FIG.10. The commercial bilayer material consisted of a macroporous layertreated by the conventional PTFE coating process and a microporous layerconsisting of a mixture of carbon powder and PTFE. The commercialbilayer was obtained from SGL Carbon (Product No. SGL 35BC). The resultsshow that the fuel cell that had the cathode made of direct fluorinationbilayer gas diffusion material delivered much better performance (morethan 50% higher peak power) than that with conventional material.

All publications, patents, patent applications, databases, and otherreferences cited in this application are incorporated by reference intheir entirety as if each individual publication, patent, patentapplication, database, or other reference were specifically andindividually indicated to be incorporated by reference.

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From the foregoing it will be seen that this invention is one welladapted to attain all ends and objectives herein-above set forth,together with the other advantages which are obvious and which areinherent to the invention. Since many possible embodiments may be madeof the invention without departing from the scope thereof, it is to beunderstood that all matters herein set forth or shown in theaccompanying drawings are to be interpreted as illustrative, and not ina limiting sense. While specific embodiments have been shown anddiscussed, various modifications may of course be made, and theinvention is not limited to the specific forms or arrangement of partsand steps described herein, except insofar as such limitations areincluded in the following claims. Further, it will be understood thatcertain features and subcombinations are of utility and may be employedwithout reference to other features and subcombinations. This iscontemplated by and is within the scope of the claims.

What is claimed and desired to be secured by Letters Patent is asfollows:
 1. A method for making a gas diffusion device comprising:providing a porous carbon substrate having a first major surface and asecond major surface which is positioned opposite to said first majorsurface and an interior between said first and second major surfaces,wherein said interior comprises interior pores with interior poresurfaces; directly fluorinating the porous carbon substrate withfluorine gas; wherein said interior pore surfaces are fluorinated;wherein at least one of the first major surface and the second majorsurface is not substantially fluorinated.
 2. The method of claim 1wherein said direct fluorinating step occurs at a temperature of about300 to 400° C.
 3. The method of claim 1 wherein said direct fluorinatingstep occurs for about 30 minutes to 3 hours.
 4. The method of claim 1,further comprising: protecting at least one of the first major surfaceand second major surface by contacting at least one surface with aprotective layer; removing said contact between said protective layerand said at least one surface.
 5. The method of claim 4 wherein saidfirst major surface is protected by contacting the first major surfacewith a first protective layer and said second major surface is protectedby contacting the second major surface with a second protective layersuch that the porous carbon substrate is sandwiched between the firstprotective layer and the second protective layer.
 6. The method of claim4 wherein said protective layer is comprised of the same materialcomprising the porous carbon substrate.
 7. The method of claim 5 whereinsaid first protective layer and the second protective layer are bothcomprised of the same material comprising the porous carbon substrate.8. The method of claim 1 in which said porous carbon substrate comprisesa macroporous carbon substrate adjacent a microporous carbon substratesuch that the gas diffusion layer is a bilayer having said first majorsurface comprised of the microporous carbon substrate and said secondmajor surface comprised of the macroporous carbon substrate.
 9. Themethod of claim 1, wherein said direct fluorination step comprisescontacting said interior pore surfaces with fluorine gas.
 10. The methodof claim 1, wherein all of said interior pore surfaces are covered withfluorine.
 11. The method of claim 1, wherein the porous substrate iscomprised of carbon fibers having thousands of carbon atomic layers;wherein an outer carbon atomic layer of said fibers is fluorinatedduring said direct fluorination step; and the method further comprisingremoving said outer carbon atomic layers of the carbon fibers forming atleast one of said first and second major surfaces such that the interiorof said gas device is fluorinated.
 12. The method of claim 11 whereinsaid removing step is performed by sanding the at least one majorsurface.
 13. The method of claim 11 wherein said removing step isperformed by physical plasma etching the at least one major surface withan argon plasma.
 14. The method of claim 11 wherein said removing stepis performed by reaction ion etching the at least one major surfaceusing plasma and a reactive gas.
 15. The method of claim 14 wherein saidreactive gas is SF₆.
 16. The method of claim 11 wherein said removingstep is performed by ion beam etching the at least one major surface.17. The method of claim 11 wherein said removing step comprises removingsaid outer carbon atomic layers of the carbon fibers forming both thefirst and second major surfaces such that the interior of said gasdiffusion device is fluorinated and both said first major surface andsaid second major surface are not substantially fluorinated.
 18. Themethod of claim 11 in which said porous carbon substrate comprises amacroporous carbon substrate adjacent a microporous carbon substratesuch that the gas diffusion layer is a bilayer having said first majorsurface comprised of the microporous carbon substrate and said secondmajor surface comprised of the macroporous carbon substrate.
 19. Themethod of claim 11, wherein said direct fluorination step comprisescontacting said interior pore surfaces with fluorine gas.
 20. The methodof claim 11, wherein all of said interior pore surfaces are covered withfluorine.